1. Field of the Invention
The present invention relates generally to a spin valve magnetoresistive effect magnetic head and a magnetic disk drive and, more particularly, to a spin valve magnetoresistive effect magnetic head, in which a change of a signal magnetic field read from a magnetic medium is converted into a change rate of an electric resistance according to a spin valve magnetoresistive effect, and a magnetic disk drive with the spin valve magnetoresistive effect magnetic head.
2. Description of the Prior Art
In a magnetic sensor or a magnetic head, a magnetoresistive effect device made of NiFe is used as magnetic material. Because of the requirement of a higher sensitivity for the magnetic sensor and the magnetic head, a giant magnetoresistive (GMR) film has drawn attention to obtain a reading signal having a high intensity. A spin valve magnetoresistive effect film serving as a type of GMR film has particularly drawn attention because the film can be relatively easily produced and a change rate of an electric resistance in the film placed in a low magnetic field is larger than that in a normal magnetoresistive (MR) device.
A magnetoresistive effect magnetic head operated according to a spin valve magnetoresistive effect has been disclosed in U.S. Pat. No. 5,206,590, U.S. Pat. No. 5,159,513, etc.
An example of a conventional magnetoresistive effect magnetic head is shown in FIG. 1A, and a cross-sectional view of the conventional magnetoresistive effect magnetic head is shown in FIG. 1B.
As shown in FIGS. 1A and 1B, an A1.sub.2 O.sub.3 TiC substrate 1, an alumina layer 2, a under layer 3 made of tantalum (Ta), a free (or unpinned) magnetic layer 4 composed of an NiFe layer 4a and a Co layer 4b, a nonmagnetic metal layer 5 made of Cu, a pinned magnetic layer 6 made of Co and an antiferromagnetic layer 7 made of FeMn are arranged in a film thickness direction in that order to manufacture a spin valve MR device. In this case, the film thickness direction is expressed in a Z direction.
The layers of a multilayered structure ranging from the under layer 3 to the antiferromagnetic layer 7 are respectively patterned in a planar rectangular shape, and a pair of electrode terminals 8 made of Au are arranged in a pair of regions placed at both edges of the antiferromagnetic layer 7 which is formed as an uppermost layer of the multilayered structure. A region of the multilayered structure placed between the electrode terminals 8 functions as a signal detecting region (or a sensing region) S. A shorter side of the rectangularly shaped pattern is directed in an X direction, and a longer side of the rectangularly shaped pattern is directed in a Y direction.
In the pinned magnetic layer 6, an exchange coupling magnetic field Hua is generated in an easily magnetization axis direction (X direction) because of an exchange coupling between the pinned magnetic layer 6 and the antiferromagnetic layer 7. Therefore, the pinned magnetic layer 6 is fixedly magnetized in the X direction, so that a magnetization direction of the layer 6 is not changed by an external signal magnetic field Hsig directed in the X direction.
A free magnetic layer denotes a magnetic layer in which a direction of the magnetization M is easily changed by the signal magnetic field Hsig, and a pinned magnetic layer denotes a magnetic layer in which a direction of the magnetization M is not easily changed by the external signal magnetic field Hsig as compared with the magnetization of the free magnetic layer.
The reason that the free magnetic layer 4 is composed of two layers represented by the NiFe layer 4a and the Co layer 4b is as follows.
First, an output of a magnetoresistive effect obtained in the free magnetic layer 4 is two or more times as high as that obtained in a free or unpinned magnetic layer made of only NiFe. Second, the Co layer 4b functions as a buffer layer to prevent an interfacial diffusion of Co and NiFe between the nonmagnetic metal layer 5 and the NiFe layer 4a caused when the layers 4a and 5 are heated.
The Co layer 4b itself is a semi-hard magnetic film. However, a two-layer structure 4a, 4b of NiFe and Co magnetically softens the layer 4 because of an exchange coupling between the NiFe layer 4a and the Co layer 4b. Therefore, the two-layer structure makes the layer 4 function as a free or unpinned magnetic layer.
The magnetization M of the free magnetic layer 4 having the two-layer structure coincides with the direction which intersects orthogonally with the magnetization direction of the pinned magnetic layer 6 (i.e., Y direction or easily magnetization axis direction) if a strength of the signal magnetic field Hsig is made to be zero. The direction of the magnetization M in the free magnetic layer 4 changes in accordance with the signal magnetic field Hsig.
A total electric resistance of the multilayered structure ranging from the under layer 3 to the antiferromagnetic layer 7 changes in proportion to a cosine of an angle .theta.(cos .theta.) between the magnetization direction of the pinned magnetic layer 6 and the magnetization direction of the free magnetic layer 4. A constant current passes through the signal detecting region (or the sensing region) S placed in the multilayered structure from one electrode terminal 6 to the other electrode terminal 8. When the total electric resistance is changed, a voltage difference between the electrode terminals 8 changes. The change of the total electric resistance is calculated according to the Ohm's law by detecting the change of the voltage difference.
The reason that the magnetization direction of the pinned magnetic layer 6 and the magnetization direction of the free magnetic layer 4 are perpendicular to each other is to make the total electric resistance linearly change with respect to the signal magnetic field Hsig.
To prevent the interfacial diffusion of Co and NiFe between the nonmagnetic metal layer 5 and the NiFe layer 4a caused by heating the layers 4a and 5, it is required that the thickness of the Co layer 4b is set to 30 .ANG. (angstroms) or more.
The inventor has experimentally examined the relationship between an external magnetic field and a change rate of electric resistance in the spin valve MR device in which the film thickness of the Co layer 4b composing the free magnetic layer 4 is set to 30 .ANG. or more.
A first sample used for an experiment has the same layer configuration as that of the spin valve MR device shown in FIGS. 1A and 1B. That is, a film thickness of the under layer 3 made of Ta is 50 .ANG., a film thickness of the NiFe layer 4a is 20 .ANG., a film thickness of the Co layer 4b is 55 .ANG., a film thickness of the nonmagnetic metal layer 5 made of Cu is 32 .ANG., a film thickness of the pinned magnetic layer 6 made of Co is 55 .ANG., and a film thickness of the antiferromagnetic layer 7 made of FeMn is 150 .ANG..
As a result of the experiment, magnetoresistive effect characteristics (.DELTA.MR) shown in FIG. 2 are obtained. This MR effect characteristics are obtained by continuously increasing and decreasing the strength of an external magnetic field in a range from -200 oersteds (Oe) to 200 oersteds (Oe). As shown in FIG. 2, it is realized that a strength difference in the external magnetic field between leading and trailing edges of a characteristic curve placed in the neighborhood of a specific point at which the strength of the external magnetic field is zero is about 40 oersteds (Oe). Because the strength difference is equivalent to a value twice as high as a coercive force Hc of the spin valve MR device, the coercive force Hc of the first sample is no less than 20 oersteds (Oe), and the sensitivity of the spin valve MR device for the signal magnetic field Hsig is degraded.
The reason that the coercive force Hc is increased to 20 oersteds (Oe) is because the free magnetic layer 4 has the Co layer (55 .ANG.) 4b with a thick film. Therefore, the inventors have tried to decrease the coercive force Hc of the spin valve MR device by thinning the Co layer 4b.
A second sample used for the experiment has the same layer configuration as that of the spin valve MR device shown in FIGS. 1A and 1B. That is, a film thickness of the under layer 3 made of Ta is 50 .ANG., a film thickness of the NiFe layer 4a is 55 .ANG., a film thickness of the Co layer 4b is 20 .ANG., a film thickness of the nonmagnetic metal layer 5 made of Cu is 26 .ANG., a film thickness of the pinned magnetic layer 6 made of Co is 55 .ANG., and a film thickness of the antiferromagnetic layer 7 made of FeMn is 150 .ANG..
As a result of the experiment, the MR effect characteristics shown in FIG. 3 have been obtained, and the coercive force Hc of the spin valve MR device has been decreased to 6 oersteds (Oe), and a change rate .DELTA.MR of an electric resistance in the spin valve MR device has been increased as compared with that shown in FIG. 2.
However, if the Co layer 4b is thinned to less than 20 .ANG., the interfacial diffusion is caused between the nonmagnetic metal layer 5 and the free magnetic layer 4 when they are subjected to heating, and it is likely to cause such a drawback that soft magnetic property of the free magnetic layer 4 is degraded.